1. Field of the Invention
The invention relates to a discharge lamp. The invention relates especially to a discharge lamp in which the discharge vessel is filled with mercury, a rare gas, with Ar as the main component, and bromine.
2. Description of Related Art
Recently, illumination devices for purposes of projection or image presentation apparatus have become common. Their light source is often a high pressure discharge lamp. In a liquid crystal projector, the light source is a discharge lamp of the short arc type because it approaches a point light source and because alignment adjustment is simple. In such a discharge lamp, there is a demand for high luminance.
However, in a discharge lamp with high luminance, it is regarded as disadvantageous that the temperature of the electrodes during operation becomes rather high so that, in this way, devitrification and blackening of the discharge vessel form prematurely, and that as a result a high lumen maintenance factor cannot be obtained over a long time.
As a measure for suppressing or for preventing devitrification and blackening of the discharge vessel in a discharge lamp, conventionally, the following measures have been proposed, for example, in patent specifications JP-A HEI 11-297268 (Publication 1), JP-A HEI 11-329350 (Publication 2), and JP-A 2002-75269 (Publication 3).
One measure involves the introduction of a halogen with a certain ratio into the discharge vessel (see publication 1 and the like); and in accordance with another measure the ratio of the maximum intensity of the emission spectra of hydrogen, oxygen, and compounds thereof to the intensity of the main emission spectrum of the rare gas in a glow discharge of the discharge lamp is adjusted to less than or equal to a certain value (see publication 2).
Furthermore, recently, to obtain a high lumen maintenance factor over a long time, a measure was proposed (see publication 3) in which the halogen cycle is activated, such that, by adjusting the discharge lamp, the ratio of the intensity of the spectrum of OH with a wavelength of 305 nm to the intensity of the spectrum of mercury with a wavelength of 404.7 nm in a glow discharge of the discharge lamp becomes greater than a certain value, a metallic substance which sprays from the electrodes reacts with the oxygen and the halogen which are present within the discharge vessel, and a metallic compound results and is then deposited again on the electrodes. However, in such a discharge lamp there are the following disadvantages:
On the silica glass tube comprising the discharge vessel and on the electrodes, carbon and hydrogen are dissolved or adsorbed on the surface. Furthermore, organic substances and water are adsorbed from the silica glass tube and the electrodes by the silica glass and the electrodes being exposed to the atmosphere in the production of the discharge lamp. As a result, carbon and hydrogen are introduced as impurities into the discharge vessel of the discharge lamp after production. When carbon and hydrogen are introduced into the discharge vessel, they react with oxygen within this discharge vessel, by which CO, CO2, H2O and the like are formed. Therefore, the halogen cycle does not operate smoothly. As a result, a high lumen maintenance factor cannot be maintained over a long period of time.
The intensity of the spectrum of mercury in a glow discharge is subject to variances depending on the outside temperature environment, the type of discharge and the like of the discharge lamp. Therefore, it is difficult to control this spectral intensity with high precision.